1. Field of the Invention
The present invention relates generally to a hydraulic control apparatus for industrial vehicles like a forklift. More particularly, this invention relates to a hydraulic control apparatus for use in industrial vehicles to operate an attachment like a forklift in accordance with the manipulation of an operational lever.
2. Description of the Related Art
As an operator manipulates the lift lever of a forklift, a lift cylinder expands or retracts to move the fork up or down. As a tilt lever is manipulated, the tilt cylinder expands or retracts to incline the mast. A vehicle such as a forklift is equipped with a hydraulic control apparatus for controlling the actuation of the lift cylinder and tilt cylinder.
As shown in FIG. 15, the actuations of a lift cylinder 161 and a tilt cylinder 162 of a forklift are controlled by a lift control valve 163 and a tilt control valve 164, respectively. The lift control valve 163 is manually operated by a lift lever 165, and the tilt control valve 164 is also manually operated by a tilt lever 166. The lift control valve 163 has a spool which moves in accordance with the up, neutral and down positions of the lift lever 165. The lift control valve 163 is connected via a pipe 167 to a bottom chamber 161a of the lift cylinder 161. The lift control valve 163 is connected to a hydraulic pump (not shown) via a pipe 163a and to an oil tank (not shown) via a return pipe 168b. The lift control valve 163 connects the pipe 168a to the pipe 167 when the lift lever 165 is moved to the up position, and connects the pipe 168b to the pipe 167 when the lift lever 165 is moved to the down position. When the lift lever 165 is moved to the neutral position, the lift control valve 163 disconnects the pipe 167 from the pipe 168a and the return pipe 168b, and holds a piston rod 161b at a predetermined position.
The down movement of the fork by the lift cylinder 161 is carried out as the piston rod 161b is moved down due to the pressure applied by the weight of the fork and the mast or the like. When the lift lever 165 is moved to the down position and the bottom chamber 161a of the lift cylinder 161 is connected to the oil tank, the fork moves downward even with the hydraulic pump stopped. As a third person or an operator accidentally manipulates the lift lever 165 to the down position while the forklift is not in operation (i.e., the engine is stopped or the power switch is off for a battery-driven vehicle) with the fork placed at the up position and the operation of the lift cylinder 161 stopped, therefore, the fork undesirably moves downward.
With the fork loaded, the center of gravity of the forklift moves frontward, and the moment which acts on the mast increases as the fork's position moves upward. As the mast is inclined frontward in a loaded condition, the center of gravity moves further forward, and thus the forward and backward stabilities of the forklift get lower.
If the rearward tilt angle is increased in a heavily loaded condition in order to cope with this situation, the center of gravity moves too rearward, lifting up the front wheels a little and the forklift may slip. In this respect, the frontward tilt angle and rearward tilt angle of the mast are set to predetermined values. While it is typical to set the frontward tilt angle to six degrees and the rearward tilt angle to twelve degrees, some forklifts specially designed with a high mast have the frontward tilt angle set to three degrees and the rearward tilt angle set to six degrees.
To put loads at a high place in an unloading work, the mast should be tilted forward while the fork is held at a high position. If the mast is tilted forward too much at a fast tilting speed due to some inadequate manipulation, loads may fall off or the rear wheels of the forklift may be lifted (i.e., instability in the forward and backward directions of the vehicle may occur). This compels the operator to carefully incline the mast at a low speed by such an inching manipulation as not to tilt the mast too frontward, and thus puts a great psychological burden on the operator. Further, tilting the mast forward with the fork held at a high position requires skills.
There are two main ways known to open and close the hydraulic passages of the lift cylinder and tilt cylinder in accordance with the manipulation of the lift lever and the tilt lever. One method uses a manual control valve (manual changeover valve) which is manually switched by the operation of a lever. The other one is to electrically detect the manipulation of a lever and switch an electromagnetic valve based on the detection by means of a controller (see Japanese Unexamined Patent Publication No. Hei 7-61792, for example).
In an apparatus disclosed in, for example, Japanese Unexamined Patent Publication No. Hei 7-61792, the controller controls an electromagnetic control valve independently of the operator's manipulation of the load lever. This accomplishes such control as to stop the fork in the horizontal position and control on the angle of the electromagnetic valve which is provided on the hydraulic passage of the tilt cylinder for controlling the flow rate. Regardless of the difference between the manual control valve and the electromagnetic control valve, sticking which causes over-friction between the spool and the body of the valve may occur due to thermal expansion originated from an increase in the temperature of a hydraulic fluid or foreign matter mixed in the oil which has entered between the spool and body. Even if sticking occurs, the use of the manual control valve allows the operator to accomplish valve switching by manipulating the load lever with a little stronger force. According to the electric control system, however, if there is a frictional resistance higher than the spool drive force which is determined from a predetermined current value previously set to actuate the electromagnetic valve, the actuation of the electromagnetic valve becomes disabled. Even if the lever is manipulated, therefore, the tilt cylinder may not move in that case.
As one way to avoid such a situation, a larger clearance may be secured between the spool and body of the electromagnetic valve so that sticking hardly occurs. This scheme however has its limitation, and increasing the clearance raises a new problem of leakage of the hydraulic fluid.
As the manual control system is generally used, the use of the electromagnetic-valve based system in the hydraulic control apparatus requires a considerable design change such as replacement of the manual control valve with the electromagnetic valve, and, what is more, the conventional components like the manual control valve unfortunately cannot be utilized. Moreover, the structure which uses the electromagnetic valve can carry out halt control of the fork and mast by controlling the closing of the electromagnetic valve, but requires separate electromagnetic valves for flow-rate regulation on the hydraulic passages of the fork and mast in order to control their speeds. This complicates the hydraulic circuit and control, disadvantageously.